Testing gearboxes and transmissions in a destined test rig—named also test stand or test bench—is usually the final procedure in overhaul or other maintenance operations. For aircraft transmissions, this so called acceptance test is mandatory. The test run procedures usually require subjecting the transmission to full power, meaning applying simultaneously the full torque and full speed that simulate the extreme load in service. Most high power transmissions are tested in closed loop test rigs in which the power from the output shaft is not wasted in a dynamometer or brake but routed back through various drives to the input shafts as in U.S. Pat. No. 4,615,212, while a ‘torque generator’ creates a twist in the loop which yields the torque. In general, a closed loop consist of a drive train that may include any kind of drives such as gearboxes, chain drives or hydrostatic drives and may include even nonmechanical drives such as electrical motors and generators. Turning the closed loop requires only the power wasted by friction in the system which is a small percentage of the power passing through the tested transmission and drives.
One type of torque generator is of the planetary or epicyclic type such as displayed in U.S. Pat. No. 2,981,103 and in U.S. Pat. No. 4,711,124. To apply it, the overall gearing reduction ratio in the loop including the planetary gearbox is set to unity, such that when starting to count from any point and tracing along the loop back to that point, the overall gearing ratio is 1:1. In this way, applying stationary torque to the case of the planetary gearbox induces torque in the loop. Another type of torque generator that may be inserted in the loop is a clutch, hydraulic or electrical as disclosed in U.S. Pat. No. 4,274,278. The overall reduction ratio in the loop is set to a value close to unity; the amount of difference from unity determines the percentage of slippage and energy loss in the clutch.
The drives that are used to close the loops are usually gearboxes. Using an identical ‘slave’ gearbox to the one being tested is the common way to achieve the gearing ratio of 1:1 in the loop. The slave transmission runs either in a reverse torque mode or in a reverse rotation direction mode relative to the tested one. The geometry of the gearbox is of great significance in this situation. When the input and output shafts of the gearbox are parallel to each other, the tested and slave gearboxes can be connected directly as disclosed in U.S. Pat. No. 3,078,711. For a transmission having the input shaft at right angle with the output shaft, the loop is usually closed in a square formation with one slave and two additional right angle gearboxes, or two additional slave transmissions.
For gearboxes with complex geometry, that is, having input and output shafts positioned, oriented at odd angles and turning in different directions, such as in many main helicopter transmissions, special matching gearboxes need to be constructed to close the loop. These specially designed gearboxes manufactured as one-off items are very expensive to build, in particular for aircraft transmission testing rigs. Few companies have the capability and experience to build gearboxes with spiral bevel gears of any shaft angle that run at speeds up to or above 20,000 rpm or gears for torques over 6000 kg-m that transmit thousands of horsepower. The test rigs of these transmissions as disclosed in U.S. Pat. No. 6,047,596 are also very large and massive relative to the tested transmission. An alternative method is using an electrical test rig as demonstrated in U.S. Pat. No. 8,758,184, where huge electrical motors and generators are used to close the power loop. These test rigs also use custom-built transmissions and are even more massive and expensive than their mechanical counterparts, but can be adopted more easily to run different kinds of transmissions.